The known prior art is summarized below in the context of electron-beam microlithographic systems. Whereas electron-beam microlithography potentially is a highly accurate method for performing pattern transfer, conventional experience with this technique has been plagued by, among various problems, low "throughput" (number of wafers that can be exposed per unit time).
Various approaches (termed "partial-pattern transfer" techniques) have been investigated to increase throughput. Examples include "cell projection," "character projection," and "block exposure." In these approaches, the entire pattern is not exposed in one shot. Rather, multiple shots are used to expose the entire pattern and only a portion of the pattern is exposed in each shot. For example, cell projection is especially used whenever the pattern comprises a small basic unit portion (measuring, e.g., (5 .mu.m).sup.2 on the wafer) that is repeated a large number of times in the pattern, such as a pattern for a memory chip in which the unit portion is a single memory cell. An image of a single unit portion is transferred to the wafer per shot; hence, many shots are required to transfer all the unit portions in the pattern. The same unit portion can be defined in multiple regions on the reticle. Unfortunately, circuit patterns such as memory chips include portions that are not repeated, and transfer of such portions requires application of another technique such as "variable-shaped beam" lithographic writing. The need to use multiple techniques to achieve transfer of the entire pattern reduces throughput.
Another approach (termed "full-field transfer"), in which a reticle defining an entire pattern is transferred in one shot to a corresponding die on the wafer, offers prospects of very high throughput. Unfortunately, however, the very large exposure field required necessitates using electron optics having a correspondingly extremely large field. Such large electron-optical systems are prohibitively costly and bulky. Also, in such large fields, the peripheral regions of the field as projected tend to exhibit large aberrations that have been impossible to date to adequately correct. Furthermore, a reticle for use with full-field exposure is extremely difficult to fabricate.
In another approach (termed "divided-pattern projection-exposure"), a reticle defines an entire pattern to be transferred to a corresponding die on the wafer, but the pattern field as defined on the reticle is divided into multiple "exposure units" (e.g., "subfields") that are individually and sequentially illuminated. Illumination is performed by an "illumination beam" passing through an "illumination-optical system" located upstream of the reticle. An image of the illuminated exposure unit passes (as a "patterned beam") through a "projection-optical system" located between the reticle and the wafer. The projectionoptical system has a field that is much smaller than the field of the entire pattern as defined on the reticle. The image that is projected by the projection-optical system onto a corresponding region of the wafer is "demagnified" or "reduced," by which is meant that the image is smaller (usually by an integer "demagnification ratio" such as 1/4 or 1/5) than the corresponding exposure unit on the reticle. Systems that perform divided-pattern projection-exposure achieve lower throughput than the full-field exposure technique but substantially higher throughput than partial-pattern techniques such as the cell projection technique.
Among the techniques summarized above, the divided-pattern projection-exposure technique has received the greatest recent attention. In divided-pattern projection-exposure, as each exposure unit is illuminated for exposure, certain dynamic imaging corrections can be made, such as correcting image focus and certain aberrations, as the image of the exposure unit is exposed on the wafer. The images are formed on the wafer surface in respective locations, selected by appropriate movements of the reticle stage and wafer stage as well as by beam deflection, that serve to "stitch" together the complete pattern on the exposed surface. Thus, exposure across an optically wide field is accomplished with better resolution and accuracy than obtained with full-field exposure.
Incidentally, in any conventional charged-particle-beam (CPB) microlithography system, whenever the current of the beam used to form the image is high, many interactions occur between individual charged particles of the beam. Such interactions, termed the "Coulomb effect," can adversely affect the quality of the image. In divided-pattern projection-exposure, undesirable Coulomb-effect changes also can be evident from one exposure unit to the next depending on differences in feature distribution and/or feature density from one exposure unit to the next. Conventional CPB microlithography systems can perform correction of some of these effects using optical-correction functions of the CPB optical system. (the "CPB optical system" is the illumination-optical system together with the projection-optical system.) For example, in a variable-shaped beam microlithography system, correction of focus of an image can be estimated from the transverse area of the shaped beam and other parameters such as acceleration voltage, current density of the beam, beam-aperture angle, and axial length of the CPB optical system.
With a conventional divided-pattern projection-exposure system, the dimensions of a single exposure unit on the reticle are (100 .mu.m).sup.2 to (1,000 .mu.m).sup.2. Conventional wisdom holds that, in the absence of other contributing factors, the Coulomb effect is insubstantial with such beam dimensions. (See, Berger et al., "Particle-Particle Interaction in Image Projection Lithography, J. Vac. Sci. Technol. B11(6):2294, November/December 1993.) This is considered to be a substantial advantage with divided-pattern projection-exposure systems because it theoretically allows beam current to be increased to obtain higher throughput without imparting excessive adverse changes to the images formed on the wafer. However, most if not all patterns that can be transferred using divided-pattern projection-exposure are not uniform. I.e., the pattern typically has a non-uniform distribution of features; even individual exposure units typically have pattern-feature distributions that are not uniform. A nonuniform distribution of features in an exposure unit will result in corresponding localized variations in the current density of the patterned beam as projected onto the wafer. The resulting corresponding variations in localized Coulomb effects can cause significant variations, over each such exposure unit, of imaging characteristics such as shape astigmatism and imaging astigmatism, as well as focus, rotation, magnification, and position of the image of the exposure unit on the wafer.
By way of example, FIG. 5(A) shows an ideally shaped image of a rectangular exposure unit, and FIGS. 5(B)-5(D) depict (in exaggerated form) respective changes in the image arising from various respective aberrations. Image-shape astigmatism changes the magnification of the image depending on the orientation, as shown in FIG. 5(B), transforming the ideal rectangular image into a parallelogram. A magnification change alters the dimensions of the image regardless of the orientation, as shown in FIG. 5(C). Rotation causes the image to be rotationally displaced about the optical axis, as shown in FIG. 5(D).
Certain of these faults can be corrected by appropriate dynamic corrections of the operational parameters of the projection-optical system. The respective magnitudes of such corrections are determined based on data concerning the reticle pattern and on data concerning various structural and/or configurational parameters of the CPB optical system. However, incorporating the respective magnitudes of correction, based on such data, with other data concerning the projection-exposure apparatus itself is impractical due to the large quantity of data that must be processed before each shot. For example, if: (1) the area of an exposure unit is (250 .mu.m).sup.2, (2) the exposure unit is divided into 16 subregions (arranged in a 4.times.4 matrix), and (3) the distribution of features in a subregion is represented by two different parameters, then there would be 2.sup.16 correction data to be calculated per exposure unit before making an exposure of the exposure unit. If corrections are made to each of shape astigmatism, imaging astigmatism, focus, rotation, magnification, and image position, then the quantity of data to be processed is further multiplied by six. In addition, correction would have to be changed whenever general apparatus parameters such as current density or beam aperture angle were changed, thereby further multiplying the quantity of data that would need to be processed.